42 CFR §406.20

1. (a)General provisions. Hospital insurance benefits are available to most individuals age 65 or over and to certain individuals under age 65 who do not qualify for those benefits under subpart B of this part and are willing to pay a monthly premium. This is called premium hospital insurance.
2. (b)Eligibility of individuals age 65 or over to enroll for premium hospital insurance. Any individual is eligible to enroll for Medicare Part A if he or she—
   1. (1)Has attained age 65;
   2. (2)Is a resident of the United States and is either—
      1. (i)A citizen of the United States; or
      2. (ii)An alien lawfully admitted for permanent residence who has resided in the United States continuously for the 5-year period immediately preceding the month in which he or she meets all other requirements;
   3. (3)Is not eligible for Part A benefits under subpart B of this part; and
   4. (4)Is entitled to supplementary medical insurance (Part B of Medicare) or is eligible and has enrolled for it during an enrollment period.
3. (c)Eligibility of individuals under age 65 to enroll for premium hospital insurance. An individual who has not attained age 65 is eligible to enroll for Medicare Part A if he or she meets the following conditions:
   1. (1)Has been entitled to Medicare Part A (under § 406.12 or § 406.15) on the basis of entitlement or deemed entitlement to social security disability benefits, as provided under section 226(b) of the Act.
   2. (2)Continues to have a disabling physical or mental impairment.
   3. (3)Loses entitlement to disability benefits (and therefore also loses entitlement to Medicare Part A under § 406.12) solely because his or her earnings exceed the amount allowed under the social security regulations pertaining to “substantial gainful activity” (20 CFR 404.1571-404.1574); and
   4. (4)Is not otherwise entitled to Medicare Part A.